Electrical device casing materials

ABSTRACT

An electrical device; e.g., a transformer, with a fluctuating magnetic field, contained within a casing comprised of steel which has a resistivity in excess of about 65 microhm-cm at room temperature and which is substantially nonmagnetic after a cold reduction of up to 65 percent. The steel consists essentially of 0.15-1.1% C, 20-40% Mn, 7-16% Al, up to 10% Ni, balance Fe and incidental impurities.

United States Patent Seitanakis Aug. 14, 1973 ELECTRICAL DEVICE CASING3,111,405 11/1963 Cairn 75/124 MATERIALS 75 1 tor: Geor e P. SeitanakisNatr 1 man ueiggts, Ona Primary ExaminerHyland Bizot Attorney-Robert F.Dropkin [73] Assignee: Allegheny Ludium Industries, Inc.,

Pittsburgh, Pa.

[22] Filed: May 20, 1971 [21] Appl. No.: 145,561 AB TRACT Related US.Application Data [62] Division of Ser. NO. 849,610, Aug. 11, 1969, Pat.No. elecmcal .devlce; flucm' $629,760 atmg magnetic field, containedwithin a casing comprised of steel which has a resistivity in excess ofabout 52 us. Cl 336/90, 75/124, 75/123 N 65 micmhm'cm tempmm" and whichis 51 Int. Cl. H011 27/02 stamiany mnmagnefic a "ductkm of "P 581 Fieldof Search 75/124; 336/90 65 Pacemsteel mists essentially 0-154 1% C,20-40% Mn, 7-16% A1, up to 10% Ni, balance Fe [56] References Cited andincidental impurities.

UNITED STATES PATENTS 2,138,754 11/1938 Andrus 336/90 2 Claims, N0Drawings The present. inventionrelates to electrical devices 5 with afluctuating magnetic field contained within casings comprised of steelwhich does not, become excessiv ely heated during operation of thedevice and more particularly to transformers contained within casingscomprised of steel which does not become excessively l heated duringOperation. of the transformer.

An improper choice of casing material can'red ce the efficiency of an.electrical device. with a fluctuating magnetic field; i.e., an AC deviceor a DC device wi h a nonsteady current flow. Poorly chosen materialsbe- L5.

come excessively heated causing an increase in the am.- bienttemperature of the deviqe... This increase in ambient temperatureresults in higher energy losses, thereby diminishing the devicesperformance.

Choice of easing materials is especially important in .0

large transformers which operate at high current levels; e.g., largepower and distribution transformers. Be.- cause of this, the presentinvention will be described with particular reference to transformerembodiments.

1t is, however, emphasized that the concepts embraced 2 by the inventionare equally applicable to all electrical devices with a fluctuatingmagnetic field.

Th exc ive heating referred to above is ca s by two. factors. The firstand most significant facto i magnetic induction which induces a oltagein the material. .3 1

It can be expressed by the equation P E1 or; by the equation F l-R'since E 1R. The equation ,9 1 R shows that heating is. influenced by thesecond. power of current. The second factor is the eddy currents whoseeffect is limited by the rcsistivityof the material.

Prior to the present; invention, transformer casings; i.e., transformercovers, were comprised of alloys which were presumed to.b.e nonmagnetic,such as AIS! type 304 stainless steel and certain manganese nickel ironalloys; e.g., alloys with 10.5 12.5 percent manganese, 7 8.5 percentnickel, balance iron and incidental impurities. l have discovered thatthose alloys are not reliably nonmagnetic and are not suitable casingsfor large transformers; i.e., transformers operating at currents inexcess of 10,000 amperes, as they form ferromagnetic martensite whencold worked.

It is accordingly anobject of this invention to provide electricaldevices with afluctuating magnetic, field contained within reliablynonmagnetic steel casing materi.- als.

It is an additional object of this invention to provide transformerscontained within reliably nonmagnetic steel casing materials.

The article of this invention is an electrical device with a fluctuatingmagnetic field contained within a casing comprised of steel. The steelhas a resistivity in excess of 65 microhm cm, preferably in, excess of130 microhm cm, at room temperature; i.e., 68F, and is substantiallynonmagnetic; i.e., the steel has a permeability of less than 1.3,preferably 1.1, at 20011 (oersteds), at cold reductions of up to 65.percent.

Illustrative casing materials which meet the require,- ments of thisinvention are given below in Table l.

The alloys of Table I differ from the previously used casing materialsinsofar as they are reliably nonmagnetic; i.e., they are substantiallynonmagnetic at cold reductions of up to 65 percent. Additionally, theyminimize the heating caused by eddy currents sincethey each have aresistivity in excess of 65 microhm cm at room temperature. Alloy C ispreferred over alloys A and B as, it has a resistivity at roomtemperature in excess of 13.0 microhm cm. A typical alloy within thealloy C range consists of 0.9 0.95% C, -26% Mn, 9,-10% A1, balance ironand incidental impurities. It has a resistivity of about 14.5 microhmcm.

A number of cold rolling tests were conducted to demonstrate thedifference between the casing materials of this invention and those usedin the past. Cold rolled were an AlSI' type 304 alloy, a manganesenickel iron alloy of the type described above and an alloy which meetsthe requirements of this invention. Typical analyses for these alloys isgiven below in Table 11.

TABLE 11 Fe and Alloy Composition Incidental C Mn Cr Ni N, Impurities D0.081.75 18.0 8.5 Ba] E 02511.5 0.5 7.25 Bal F 0.1 8.150 17.0 1.25 0.35Bal.

Alloy D is the. type 304 alloy, alloy E is the manganese nickel. ironalloy and alloy F is the alloy which meets the. requirements of theinvention.

Found below in Table 111 are the results of the cold rolling tests.

TABLE, 111

Alloy D Alloy E Alloy F Cold (Permeability (Permeability (PermeabilityReduction At 200 H). At 200 H') At 200 H) 0 L015 1.01 1.002 10 1.0641.32 1.002 3.235. 1.74 1.003 5.0, 8.480 1.90 1.003 14.410 1.95 1.004

'The difference in the casing materials of this invention and those usedin the past is readily seen from Table III. A study of the resultsreveals that only alloy F remainss substantially nonmagnetic at coldreductions of up to about 65' percent. lts permeability was less than1.10 at 200 11 (oersteds) after a cold reduction of 65 percent. Forcomparison purposes the permeability of air is Alloys D and E becamesomewhat ferromagnetic due to the formation of martensite. Although thepermeability of alloy E is tolerable at low currents and magneticfields, it is. not tolerable at the very high currents and magneticfields found in the large transformers used today; i.e., transformers of600 KV and greater. in these large transformers small amounts offerromagnetitmfcan have a strong influence on case heating. The effectof coldworking, exemplified by the above described tests, is compounded.by' the fact that casings generally undergo the most severe cold workingin the area which surrounds the bushings, the area most subject toexcessive heating.

It will be apparent to those skilled in the art that the novelprinciples of the invention disclosed herein in TAB LEl Composition Feand Incidental Alloy ,C Mn Si Cr Ni N M0 Al Impurities A 0.12-0.2514-15.5 0.5 (max) 16.5-18.0 l.00 -1.7-5.0.3-2-0.40 Bal. B 0 08 (max)2.00 (max) 1.00 (max) 16.0-18.0 10.0-14.0 2.0-3.0 Bal. C 0.151.l 20-40 10.0 (max) 7-16 Bal.

connection with specific examples thereof will suggest various othermodifications and applications of the same. It is accordingly desiredthat in construing the breadth of the appended claims they shall not belimited to the specific examples of the invention described herein.

I claim:

1. An electrical device with a fluctuating magnetic field, containedwithin a casing comprised of steel which has a resistivity in excess ofabout 65 microhmcm at room temperature and a permeability of less than2. An article according to claim 1 wherein said steel consistsessentially of from 0.90.95% C, 25-26% Mn, 9-l0% Al, balance Fe andincidental impurities.

2. An article according to claim 1 wherein said steel consistsessentially of from 0.9-0.95% C, 25-26% Mn, 9-10% Al, balance Fe andincidental impurities.